[LLVMbugs] [Bug 12850] New: Severe constexpr compile-time performance issue

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=12850

             Bug #: 12850
           Summary: Severe constexpr compile-time performance issue
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: andyg1001 at hotmail.co.uk
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified


The following is some simple test code that causes clang to slow to a crawl:


#include <stdio.h>

constexpr int fn(int i) {
    return i < 0 ? 0 : (i + fn(i-1) + fn(i-2));
}

constexpr int e = fn(NUM);

int main() {
    printf("%i\n", e);
    return 0;
}


With gcc 4.6.3 or 4.7.0, compiling using "gcc -std=c++0x test.cpp -DNUM=???"
(where NUM is given in the table below), takes the following compile times:

NUM | compile time | output
================================
  5 |     0.03     |  26
 10 |     0.03     |  364
 15 |     0.03     |  4163
 20 |     0.03     |  46345
 25 |     0.03     |  514201
 30 |     0.03     |  5702854
 35 |     0.03     |  63245948
 40 |     0.03     |  701408690

On clang 3.1 rc-1, the corresponding compile times become:

NUM | compile time | output
================================
  5 |      0.03    |  26
 10 |      0.04    |  364
 15 |      0.04    |  4163
 20 |      0.12    |  46345
 25 |      1.03    |  514201
 30 |     11.16    |  5702854
 35 |    123.93    |  63245948
 40 |   1377.85    |  701408690

In both cases, compile times are in seconds.  I can only assume gcc must be
somehow caching previously evaluated function calls...

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